1. Field of the Invention
The subject invention relates to flexible printed circuit fabrication and, more particularly, to a method for defining and building raised metal features for use in connection applications.
2. Description of Related Art
Various applications exist where it is desirable to form raised metal connection features above a surface. One such application is in the formation of connector assemblies wherein such raised features form interconnection contact points on flexible structures, for example, as illustrated in U.S. Pat. No. 4,453,795 assigned to Hughes Aircraft Company. According to the disclosure of that patent, a first wafer has copper contact pads on which are formed raised metal connector features in the form of copper buttons. A second wafer has copper pads located to mate with the copper buttons. The first and second wafers are then aligned and compressed together to establish connection between the copper buttons of the first wafer and the pads of the second wafer.
The conventional method of creating raised metal connection features, such as the copper buttons of the '795 patent, requires multiple photolithographic steps. First, photoresist is laminated, exposed and developed three times to create a circuit with an interconnection feature which extends above an acrylic adhesive dielectric film substrate surface 0.002 to 0.003 inches. Then photoresist is "tented" around the raised feature and another photoetch step is performed to define circuitry on a copper pad located on the substrate. Finally, a predrilled coverlay is applied over the circuitry, and gold plating is applied to the areas exposed by the coverlay to define the feature.
The foregoing process has several problems. Numerous steps are required, resulting in a labor-intensive procedure. Second, trying to tent photoresist around the raised feature is difficult and frequently results in voids. Additionally, the raised feature holds the coverlay off the copper pad, resulting in poor line definition. Finally, alignment of a predrilled coverlay is difficult and still leaves exposed copper on the pad area which must be gold plated. Gold plating degrades exposed areas of the acrylic adhesive substrate.